1. Technical Field
The present invention relates to a driving circuit for a capacitive load and a liquid injecting apparatus, and more particularly, to an ink jet record head that drives a piezoelectric element by using a driving signal having a trapezoidal shape and an ink jet recording apparatus having the ink jet record head.
2. Related Art
As liquid injecting apparatuses that perform a printing process or the like by injecting a liquid in accordance with supply of a driving signal and landing the liquid in a target, for example, ink jet recording apparatuses including an ink jet record head that injects ink droplets from a nozzle opening by using pressure that is generated by displacement of a piezoelectric element have been known. In the liquid injecting apparatus of this type, supply of a sufficient current is needed so as to operate a plurality of the piezoelectric elements without any problem. Accordingly, a driving signal of which current is amplified by a current amplifying unit is used.
When the current of the driving signal is amplified by the current amplifying unit, the power consumption of a charging transistor is an amount acquired from multiplying the current by a difference between the electric potentials of the power source and the driving signal. On the other hand, the power consumption of a discharging transistor is an amount acquired from multiplying a current by a difference between the electric potential of the driving signal and the ground electric potential. Accordingly, the power consumption of each transistor is increased, and thus, technology for decreasing the power consumption as possibly as can be has been requested. As general technology for decreasing the power consumption due to the driving current in response to the request, technology has been disclosed in JP-A-2006-272907.
In the driving circuit disclosed in JP-A-2006-272907, for a main driving signal having a trapezoidal shape that is used for driving a piezoelectric element, an auxiliary driving signal that is offset by a predetermined amount so as to follow the shape of the main driving signal is generated, and the auxiliary driving signal is used as the power-source voltage. Accordingly, the difference between the main driving signal and the power-source voltage is decreased, and whereby the power consumption is reduced.
Thus, the driving circuit includes a main driving signal generating unit that generates a main driving signal through the transistor pair based on an analog signal and an auxiliary driving signal generating unit that generates an auxiliary driving signal through other transistors and a smoothing circuit based on a pulse signal. The pulse signal is acquired from comparing a signal representing the main driving signal generated by using a PWM circuit and a chopping wave by using a comparator.
However, in the PWM circuit of the driving circuit disclosed in JP-A-2006-272907, for the signal representing the main driving signal to be compared with the chopping wave, a process for adding a specific value to the main driving signal so as to add an offset to the main driving signal or the like is performed. Accordingly, the adjustment operation for adding a predetermined offset is troublesome. In addition, a difference between the main driving signal and the auxiliary driving signal may decrease due to delay of the smoothing circuit or the like, and in such a case, the operation is unstable. Recently, the frequency of the main driving signal has been increased prominently, and thus, the influence of the delay cannot be ignored as that much. On the other hand, when the offset value between the main driving signal and the auxiliary driving signal is set to be large from the start, there is a problem that it is difficult to decrease the power consumption by reducing the heat loss of the transistor pair.